The present invention relates to expanded memory system (EMS) architectures.
An EMS system is used to increase the memory addressability beyond the logical addressability of a microprocessor. Certain portions of the address space are designated as the EMS "window". An EMS access is triggered by an address which has higher order bits indicating that it is within the EMS window. This is determined by comparing certain of the address bits to a stored register value indicating the portion of memory designated as the EMS window. Upon a match, certain of the higher order address bits are translated to give the actual physical address to be used. A diagram of such an EMS system is shown in FIG. 1, where two different blocks in the EMS window of the one megabyte memory space are mapped to different physical areas of memory in the two megabyte physical memory.
In the typical EMS system, the physical memory is DRAM (dynamic random access memory). Separate portions of memory are reserved for the ROM (read only memory) used to store the microcode or other fixed portions of the system.